We have cloned a novel orphan nuclear receptor w3hose embryonic expression pattern suggests that it is involved in regulating important developmental processes. The long term goal of this project is to understand the physiological role of the orphan nuclear receptor GCNF in embryonic development. To this end we have created a mouse model in which complete loss of GCNF function leads to embryonic lethality mid- gestation, with profound effects on many developmental processes and systems. This includes a major patterning defect involving disruption of the anterior-posterior axis causing a severe truncation caudal to the hindbrain, a halt in somitogenesis and defective differentiation of mesoderm, and failure of posterior ventral organogenesis. We hypothesize that GCNF is a developmentally important receptor and transcription factor that regulates embryonic axis formation and organogenesis by controlling the expression of developmentally important genes. To test this hypothesis we will utilize the GCNF knock- out mouse model to dissect the role of this orphan receptor in embryonic development. To achieve this goal we propose three specific aims. 1) Complete the characterization of the defective phenotype of the GCNF knockout-out embryos. The LacZ reporter gene will be knocked into the GCNF locus to aid in the characterization of the GCNF knock-out phenotype. The characterization of the patterning defects anterior- posterior axis, organogenesis abnormalities and the mesodermal differentiation defects will also be completed. 2) Subsequently, we will determine whether GCNF is acting in a cell autonomous or non- autonomous manner in each of the three germ layers to generate the observed phenotype. Using the LacZ knock-in mice, aggregation chimeras, containing GCNF (-/-) ES cells tagged with beta-Gal and wild type ES cells will be generated to answer this question. 3) We propose to identify GCNF target genes important for embryogenesis. We will determine effects of the loss of GCNF on the expression of appropriate mutant and wild type embryos. The completion of these experiments will yield greater understanding of the role GCNF plays in regulating embryonic development by controlling gene expression.